I took a photograph without the prism finder, set to 8S, and it worked. So what's a real head scratcher is why 4s works but it seems to completely ignore 8s. I guess my next test is to shoot the prism finder at 16s and see if it times it correctly. If not I can presume that the prism finder is unable to draw enough power from the batteries > 4s to close the shutter and as such is defaulting to 1/500.